8259631: Reapply pattern match instanceof use in HttpClientImpl

Reviewed-by: dfuchs, chegar

Author: AlekseiEfimov

src/java.net.http/share/classes/jdk/internal/net/http/HttpClientImpl.java

@@ -158,8 +158,7 @@ public void execute(Runnable command) {
         }
 
         private void shutdown() {
-            if (delegate instanceof ExecutorService) {
-                ExecutorService service = (ExecutorService)delegate;
+            if (delegate instanceof ExecutorService service) {
                 PrivilegedAction<?> action = () -> {
                     service.shutdown();
                     return null;